1. Field of the Invention
The present invention relates to an audio signal transmission system for use in digital devices and, particularly to a system which is capable of producing a high-fidelity output.
2. Related Background Art
Recently, in audio systems, various digital devices such as compact disc systems (CD) or digital audio tape recorders (DAT) have appeared thereby to reproduce high-quality audio signals.
Audio techniques for entertainment media have started to spread out in various fields in various manners.
FIG. 1 schematically shows the structure of a general audio device. In FIG. 1, reference character A denotes an input medium, B a transmission section including an amplifier, etc., and C an output section including a loudspeaker, etc. Recent high-quality audio devices are capable of greatly reducing transmission distortion in the transmission section B. This is in large part due to recent remarkably advanced very large scale integration (VLSI) techniques.
Improvements in tone quality using these VLSI techniques follow prevention of tone quantity deterioration due to transmission distortion in the transmission section 2. Enhancements of tone quality which follows an increase in the speed of operation of a large scale integrated (LSI) circuit element which processes digital signals in the CD or DAT, and which follows so-called emphasis processing and/or noise reduction processing is solely intended to suppress transmission distortion in the transmission path and to supply the audio signal input through an input medium A to the output section C with high fidelity using optimized material, structure, etc.
Table 1 shows the fidelity of individual elements of FIG. 1. AS is clear in this table, recently, the fidelity in the input and output sections, especially in the output section, will be greatly deteriorated and there is a large difference between a live voice and a reproduced voice from an audio device, although same may be of high quality.
Mechanical vibration systems such as microphones or loudspeakers, have mass, and the system for holding the vibration system also fulfills the function of a damper. The presence of the mass and damping will result in waveform distortion, and especially, deterioration in the, transient characteristic, and residual vibration. These disadvantages are especially conspicuous in loudspeakers which produce large energy.
For example, the band of audio frequencies is about 20 to 20,000 Hz. It is very difficult to reproduce the entire band of these frequencies with high fidelity using a single loudspeaker. Usually, a plurality of loudspeakers are used to reproduce individual frequency bands thereby to reproduce the entire band of audio frequencies. This process of dividing the input signal into frequency bands and supplying same to a plurality of corresponding loudspeakers, a so-called crossover network division, includes the following two approaches.
One is a passive network which performs division at the output stage of the power amplifier and the other is a multiway system which performs division before the input signal is input to the power amplifier. Generally, the passive network can be composed more inexpensively than the multiway system whereas the multiway system can reproduce the audio signal with more fidelity than the passive network. Generally, the multiway system is employed more often.
The scheme of the multiway crossover network system is either of an analog type which includes a combination of R, L and C elements, or of a digital type which converts the input signal to a digital signal and processes same.
In the analog multiway system, it is difficult to make decay and phase characteristics of the crossover frequency characteristic compatible. The frequency characteristics of the individual systems may not be uniform due t possible uneven characteristics of the corresponding parts included in the respective systems. On the other hand, in the digital type multiway system, the out-of-band decay and phase characteristics are compatible, but the crossover frequency is limited. Especially, a low-band crossover network cannot be realized.